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272                       STOMACH OF ARTIODACTYLES                       CHAP.
can see how this may be purely adaptive, the push of the hind-
legs in running needing a firmer support. In Hyomoschus this
is the case. The hind-limbs are provided with a cannon bone,
while the metacarpals of the fore-feet are still free.
The number of dorso-lumbar vertebrae is less in the Axtio-
dactyl© than in the Perissodactyle Ungulates. Whereas the former
have but nineteen, the latter have, as a rule, twenty-three such
vertebrae.1 The number of ribs varies from twelve {Camelus,
Hydropotes) through thirteen (Cervus, Gazella) to fourteen in
Dicotyles, Giraffa, etc.
The curious form of teeth known as " selenodont" is character-
istic of the Artiodactyla, though only found well developed in the
modern forms, and of those only in the Pecora. The more primitive
forms had " bunodont M teeth with typically four tubercles (if we
except the tritubercular and but little-known Pantolestes) ; and the
intermediate " buno-selenodont" type characterises such groups as
the Anthracotheriidae.
While the stomach of the Perissodactyles is always a simple
sac, it is complicated, or shows signs of complication, in the
Artiodactyles. That of the Hippopotamus is divided into two
chambers; there are three in Tragulus, and four in the typical
Ruminants such as Cervus, Oms, etc.
Had we to deal only with the still living genera of Artio-
dactyles, it would be easy to sort them into two groups on the
characters of the teeth; for the Pigs and Hippopotamus are pro-
vided with tubercular molars; they are bunodont. The Deer,
Camels,, Oxen, Giraffes, etc., have selenodont molars. Besides, the
latter are " Ruminants," and have a more complicated stomach.
The existing Chevrotains forbid a more trenchant division, since
they are, as will be pointed out in due course, somewhat inter-
mediate in structure ; the feet are more Pig-like, and the stomach
is not so typically Ruminant. In any case such a division is pre-
vented by certain extinct families which are perhaps ancestral to
both. They have teeth which are not quite bunodont and not
quite selenodont. These teeth have been termed buno-selenodont
or bono-lophodont.
The distribution of the living Artiodactyles presents us with
some interesting facts. The vast preponderance of species occurs
im fee Old Worldó34 in America as against over 250 species